For radio broadcasters, knowing the impedance a transmitter is operating into can be essential for ensuring proper spectral performance and reception, particularly with digital standards such as National Radio System Committee NRSC-5A In-band/on-channel Digital Radio Broadcasting Standard. The impedance may be influenced by different factors such as the transmitter filter, tuning networks external to the transmitter, and the antenna. As a result, the impedance must fall within certain specifications, or problems may develop with respect to spectral compliance or reception which is especially true for digital operation.
Currently, the equipment required to determine the impedance that the transmitter is operating into has to be inserted at the appropriate place, so the transmitter must be disconnected to effect this measurement. This results in the transmitter being temporarily taken off the air while the measurement is being made. In addition, specialized equipment that may not be commonly available at the transmitter site is required to make the measurement. As a result of these factors, the current method is time consuming, costly and can require several iterations before an acceptable operating impedance is reached.
For broadcasters operating in a single frequency network, using a training signal to measure the impedance is not a preferred option, since emitting a tone would disrupt the other transmitters in the network. This method virtually eliminates reception, therefore, the broadcaster's only option for measuring the impedance involves disconnecting the transmitter to use low power test equipment. In addition a generated test signal is typically utilized to perform impedance measurements for matching network tuning. These estimates may not accurately characterize the impedance response based upon broadcast signals propagated by the antenna providing less than optimized tuning of the matching network.
The current methods of measuring the impedance seen by a transmitter require disconnecting the transmitter, which is inconvenient, time consuming, and costly, and does not provide a ‘real-world’ antenna impedance estimation. Accordingly, there is a need to develop a method to measure impedance seen by a transmitter in an antenna without having to remove the transmitter from the network.